Thank you for your research in the matter. I did some research about 8 years ago and I just can't remember all the details and didn't bookmark them, but I seem to remember that they were using it for a preservative not as a vitamin supplement. Perhaps you will find something about that?
I know some people here hate the "Dog food adviser", but he has written a good article on the subject, that is much easier than reading these studies. He also has references in this article.
The Controversy Over Menadione in Dog Food
Vit K 1 and Vit K 2 are natural but Vit K 3 (menadione) is synthetic and all forms of the chemical must first undergo the cellular process of alkylation before they can be used by the body. It is not used directly as a nutritional supplement due to high toxicity and low stability
MENADIONE (Vitamin K3)
I think it's important to note that menadione has been banned from human supplements
Vitamin K | Linus Pauling Institute | Oregon State University
"Although allergic reaction is possible, there is no known toxicity associated with high doses of the phylloquinone (vitamin K1) or menaquinone (vitamin K2) forms of vitamin K11.
The same is not true for synthetic menadione (vitamin K3) and its derivatives.
Menadione can interfere with the function of glutathione, one of the body’s natural antioxidants, resulting in oxidative damage to cell membranes.
Menadione given by injection has induced liver toxicity, jaundice, and hemolytic anemia (due to the rupture of red blood cells) in infants12; therefore, menadione is no longer used for treatment of vitamin K deficiency13.
No tolerable upper level of intake has been established for vitamin K.”
The FDA has banned the use of menadione from over-the-counter supplements because large doses have been shown to cause these dangerous biological effects.14". Taken from the
The Controversy Over Menadione in Dog Food
The bottom line for me is if it's not safe for humans, I don't think I want it in my dog food. They can still use it in animal "feed", but my dog isn't a farm animal that will be sent to market after a certain age. Farmers don't care if they animals get cancer at 10 years old, but our motivation is to keep our dogs alive and healthy as long as possible. Many of the large food companies have voluntarily taken it out, just because some people made enough noise and consumers started buying foods without it, although you can still find it in some foods.